Selective hydrogenation with tris(triphenylphosphine) chlororhodium (I) catalyst: Preparation of octadecenoate isomers

Fractionation of products obtained from partial catalytic hydrogenation of methyl cis ‐9, cis ‐12‐octadecadienoate (9c,12c‐18:2) with tris(triphenylphosphine) chlororhodium [RhCl(Ph 3 P) 3 ] provided a facile method for preparation of a nearly equal molar mixture of methyl cis ‐9‐ and cis ‐12‐octadecenoate (9c‐18∶1 and 12c‐18∶1). Isolation of products was achieved by silver resin and C18 reverse phase liquid chromatography. Catalytic deuteration of 9c,12c‐18∶2 yields a mixture of 9c‐18∶1‐12,13‐d 2 and 12c‐18∶1‐9,10‐d 2 with an isotopic purity of 85%. Final isolated yield of the mixture of 9c‐ and 12c‐18∶1 products was 30%. Isolation of products from partial hydrogenation of conjugated octadecadienoates (9c,11t‐18∶2 or 10t,12c‐18∶2) provided a convenient method for synthesis of an almost equal molar mixture of methyl trans ‐10 and trans ‐11‐octadecenoate (10t‐18∶1 and 11t‐18∶1). Characterization of the reaction products from hydrogenation of 9c,12c‐28∶2 indicates that the 9c‐ and 12c‐18∶1 products are formed by the expected 1,2‐hydride addition. The presence of small amounts of 10t‐ and 11t‐18∶1 and conjugated octadecadienoates was evidence for a secondary isomerization‐1,4‐hydride addition pathway. Isolation and characterization of products from RhCl(Ph 3 P) 3 ‐catalyzed hydrogenation of 9c,11t‐18∶2 and 10t,12c‐18∶2 indicate that both 1,2‐ and 1,4‐hydride addition to the conjugated diene isomers occurs at about equal rates, but only the cis bond is reduced by the 1,2‐hydride addition pathway and the 1,4‐hydride addition pathway yields only a trans ‐18∶1. Because of this unusual selectivity for a cis bond conjugated with a trans bond, hydrogenation of both 9c,11t‐18∶2 and 10t,12c‐18∶2 yields the same mixture of t‐18∶1 isomers.